Problem to be investigated: The main challenges in dealing with carcinoma of the prostate and other organs are posed by metastatic disease. Unfortunately, these are not surgically or radiologically amenable, nor do they respond to systemic therapies in a manner similar to the primary lesion. Our inability to treat these deadly harbingers derives from a paucity of knowledge of carcinoma cell behaviors in the metastatic environment. In our current grant period, we found that prostate carcinoma cells change their cell behavior and phenotype in the metastatic niche; the generalizability of these findings was supported by similar behaviors (but distinct molecular controls) in breast carcinomas. To escape the primary tumor mass, these cells dedifferentiate to a more mesenchymal-like cell functioning - one of active migration and proliferation. However, in the metastatic niche, the cells appear to revert back towards a more differentiated state. This is seemingly driven by signals from the target organ. Thwarting our therapeutic approaches, such an epithelial state along with connections to the resident tissue also appears to promote cancer cell survival. Thus, we posit a foundational model of behavioral plasticity of the tumor cells as determined by cues from their environment. Hypothesis and Objectives: That carcinoma cells undergo phenotypic changes driven by the metastatic microenvironment so as to enable pathological survival during dissemination and subsequent growth. We posit that E-cadherin connections re-established (possibly transiently) during metastatic seeding promote metastatic seeding, quiescence and resistance to cell killing. These three distinct but complementary behaviors, any one of which would confound treatment, would require new thinking in our approaches to metastatic prostate cancer. We propose to test this novel foundational model of carcinoma-parenchymal communications in an innovative organotypic culture system as well as in directed culture and animal models. In Objective 1, we will determine whether the reversion to a more differentiated state at the metastatic site protects carcinoma cells from death, whether caused by inflammation, chemotherapy or cellular starvation - all challenges for a metastatic cell to overcome. This, and the molecular basis underlying it and whether E- cadherin signaling is the major contributor, will be assessed in vivo and the molecular controls elucidated in both two- and three-dimensional complex, and a unique organ culture model system. In Objective 2 we will further probe the molecular underpinnings of the redifferentiated tumor cells to establish metastases. We will focus on the E-cadherin expression and connections with non-cancer cells in the target organs and whether these are critical for micrometastases. This will be tested in animal models and a novel ex vivo liver organotypic bioreactor to better parse the carcinoma and host contributions. Significance and Innovation: The successful completion of these experiments will shed new light on molecular controls that are subverted in prostate and other carcinomas to promote progression. These studies test novel hypotheses using state-of-the-art techniques. The major impact would be on our fundamental understanding of the tumor biology at initial metastasis seeding, an understudied yet critical step in dissemination. We can probe this due to the novel tools available with the liver bioreactor. Previous tools examining established metastases may have missed the initial phenotypic plasticity that occurs during this seeding event. The translation to clinical impact would occur in discrete steps. The validation of even just parts of our foundational model would alter our understanding of initial and cryptic metastases. First, the establishment of phenotypic reversion to an epithelial, quiescent phenotype during metastatic seeding may promote use of agents that target non- and slow-cycling cells. Second, upregulation of epithelial markers during initial seeding may allow for imaging to better detect these micrometastases. Lastly, in the longer term, select molecular fingerprints or signaling cascades could be developed as targets for future agents.